1. Field of the Invention
This invention is in the field of fluidic devices and, more specifically, is related to fluidic partial pressure sensors which obtain direct indications of a gas constituent in a mixture.
2. Description of the Prior Art
A low cost partial pressure sensor would certainly be an asset in systems that require a constant partial pressure of a gas in a mixture no matter at what ambient pressures the system rests. An example of such system would be an oxygen control system needed for the control of life-support systems aboard aircraft where it is desirable to provide the ground equivalent of oxygen to the aircraft operators regardless of altitude.
Most prior art systems required the measurement of two parameters, gas concentration and ambient pressure, the product of which is, by definition, partial pressure. Therefore, in addition to the measurements, there must be a computation to determine how much oxygen is to be admixed with the ambient air. Because such systems generally required the use of two devices for measurement and computational equipment in order to schedule the correct amount of oxygen to the aircraft, these systems were not responsive to situations involving varying ambient pressures and certainly not simple.
U.S. Pat. No. 4,008,601 to Robert L. Woods proposes a solution to this problem by providing a technique for obtaining a direct indication of the partial pressure of a gas constituent in a mixture relative to a reference gas. Woods utilizes fluidic concentration sensors of the type disclosed in U.S. Pat. No. 3,771,348 to Villarroel and in U.S. Pat. No. 3,756,068 to Villarroel et al. and operates the device on the premise that the bridge pressure across the device can be scheduled to follow the ambient pressure by using an aspirator with the proper functional characteristics.
The equation which is the basis for operation in the Woods' technique is EQU .DELTA.P.sub.o = GK.sub.1 C.sub.1 (P.sub.o2) (B-1)
wherein:
.DELTA.P.sub.o = bridge pressure output PA1 G = the sensor gain constant PA1 K.sub.1 = gas sensitivity constant PA1 P.sub.o2 = partial pressure of oxygen and PA1 C.sub.1 = a constant. PA1 P.sub.b = bridge pressure drop and PA1 P.sub.a = the ambient pressure.
This equation is reached by assuming that the aspirator performance will result in EQU P.sub.b = C.sub.1 P.sub.a (B- 2)
wherein:
Woods' technique also assumes that G in equation (B-1) is constant. However, this is not necessarily so, and .DELTA.P.sub.o will not be proportional to P.sub.o2 as originally postulated. Indeed, with P.sub.b varying as shown in equation (B-2) .theta. (of which G is a function) will vary as EQU .theta. C.sub.2 P.sub.a.sup.2 (B-3)
where C.sub.2 = constant.
The effect of equation (B-3) on the value of G as P.sub.a varies from 0.3 to 1.0 atmospheres is significant. The present invention, however, makes use of the variation of G with P.sub.a to obtain the desired function of partial pressure sensors.